1. Field of the Invention
The present invention generally relates to an inductor and a manufacturing method thereof. More particularly, the present invention relates to an inductor having a spiral metal layer on an insulating film formed on a semiconductor substrate and a manufacturing method thereof.
2. Related Art
With recent popularization of cellular phones and PDAs (Personal Digital Assistances), there has been a growing demand for reduction in size of high frequency circuits having a wireless interface. Although passive elements such as an inductor used to be mounted outside a semiconductor device, such as on a printed circuit board portion (that is, passive elements used to be external parts), they have been increasingly mounted within a semiconductor device. An inductor is a part that is required for application to RFIC (Radio Frequency Integrated Circuit) design such as a LNA (Low Noise Amplifier), a PA (Power Amplifier), and an RF (Radio Frequency) oscillator. It is relatively easy to form a resistive element and a capacitive element within a chip. However, it is not necessarily easy to form an inductor within a chip because there are various inductor structures for causing inductivity.
Related art will now be described with reference to FIGS. 7A and 7B. FIG. 7A is a plan view of a spiral inductor. FIG. 7B is a cross-sectional view taken along line VIIb-VIIb′in FIG. 7A.
As shown in FIG. 7A, an inductor 20 is formed by a spiral coil-shaped inductor wiring 11. An outer terminal 11a of the inductor 20 is connected to a via 12 and a wiring 14 and an inner terminal 11b of the inductor 20 is connected to a via 13 and a wiring 15. The inductor wiring 11 is thus electrically extended to the outside. As shown in FIG. 7B, the inductor 20 (inductor wiring 11) is provided in an interlayer insulating film 10 formed on a semiconductor substrate 1, and the inductor wiring 11 has a rectangular cross section when cut in a width direction.
Characteristics of a spiral inductor will now be described. Q-factor (quality factor) is commonly used as an index indicating inductor characteristics. For example, in a series resonant LC circuit, Q-factor is determined by dividing an inductor value at a resonance frequency by a series resistance of the circuit as shown by the formula (1):Q=ωL/R  (1)where ω is 2πf, π is a ratio of the circumference of a circle to its diameter, f is a frequency, L is an inductance value, and R is a resistance value.
It is considered that a higher Q-factor inductor has better electric characteristics. A higher Q-factor inductor also contributes to reduction in power consumption of a circuit.
FIG. 8 shows typical Q-characteristics (change in Q-factor with respect to a frequency) of an inductor. As shown in FIG. 8, Q-factor at first increases with increase in frequency. As the frequency becomes higher, however, Q-factor decreases due to a capacitive loss by capacitive coupling between the inductor and a substrate. The Q-characteristics therefore have a convex wave shape.
It can be seen from the formula (1) that it is effective to reduce a serial resistance component of an inductor and to suppress a capacitive loss in order to increase Q-factor.
An example of such related art is disclosed in Japanese Laid-Open Patent Publication No. 2004-207602 (especially page 28, FIG. 25).